In a stem cell development talk this moring, Haifan Lin, director of the Yale Stem Cell Center showed a picture of a pie he had decorated with the letters R-N-A. What was he celebrating? Something that quite frankly frightens me.
His group has been investigating piRNAs. These are short strands of RNA, generally 30 nucleotides long in mice and a little bit shorter in Drosophila. These are similar in some ways to microRNAs and short interfering RNAs (siRNAs), but the name piRNA (literally pronounced “Pie-R-N-A”) comes from the fact that they interact with a protein called PIWI. They’re found in abundance in the testes of mice and flies and appear not only to stifle gene expression by interacting with PIWI and corresponding DNA sequences (similar to other short RNAs) but also to stabilize and promote expression (possibly similar to other short RNAs as well). Just distinguishing these elements from other short RNAs is tricky enough. The janus-like nature of piRNAs could make for a real headache. What dictates whether they turn things on or off? Then there’s abundance. Studies have counted as many as 50,000, and Lin estimates that there are as many as four times that amount. When he realilzed how long that would keep his lab busy, Lin brought in some RNA pie to celebrate (the picture looked like apple). Still he’s not completely comfortable with all the implications of his work. The testes produce thousands of different piRNAs all coming from DNA that was formerly considered junk. “Does that make the testis a genetic junkyard?” Lin asked. He quipped: “At least emotionally I have a problem with that.”